The invention relates to a lift system that produces a vertical parallel movement of a platform relative to a base, and it also relates to a weighing system that has a sample changer equipped with the inventive lift system that produces the vertical parallel movement of a sample changer platform.
Lift systems are used in different technical fields where a vertical movement, e.g. of a platform or an object relative to a base is required.
For example in the field of weighing technology, a lift system in conjunction with a sample changer serves to transfer one or more weighing object units or weighing containers to a balance pan. In a frequently used arrangement, the balance pan is located above or below a sample changer position of a sample changer, where the balance pan and the sample changer are vertically movable in relation to each other.
A piece-counting apparatus with a balance is disclosed in U.S. Pat. No. 5,883,336. A rotating sample changer has receiver openings for a container into which the pieces are placed that are to be counted. An empty container is brought by the sample changer into the filling position above the balance. By means of a pneumatic lifting mechanism, a weighing platform is moved vertically to lift the container off the sample changer—by reaching through the receiver opening of the sample changer—in order to perform the weighing. The lifting mechanism in this apparatus has a pneumatic lifting cylinder arranged centrally below the weighing platform and, in addition, requires three guiding elements arranged around the lifting cylinder, each of which consists primarily of a pin guided in a sleeve.
Known technical solutions are in practical use for balances, in particular for comparator balances, where a sample changer working together with the balance is equipped with a single lift unit arranged in the middle of a platform that is vertically movable in relation to the weighing pan. This device is suitable for balances that are specified for smaller loads, e.g., in a range from a few grams up to a few kilograms because, due to the small overall dimensions of the sample changer and the balance pan, the vertical travel distance between the sample changer and the balance pan for the transfer of the weighing object is likewise relatively small. It is therefore possible to use a simple mechanical device, such as for example an eccentric, as a lifter unit, although the drive torque as well as the speed of the vertical movement are not constant over the vertical lifting range of an eccentric.
For balances with a fine resolution of the weighing result, it is of critical importance that after the transfer of one or more weighing object units onto the weighing pan, the combined center of gravity of the weighing object units should lie on a vertical line passing through the area where the load is introduced into the weighing cell. It is therefore a requirement that the vertical movement, e.g. for seating the weighing object on the sample changer, occurs in exact parallel alignment relative to the weighing pan and furthermore in a sufficiently gentle and jolt-free manner so that the weighing object units will not shift their positions relative to each other during the transfer.
As a principal observation, in the case of low-capacity balances that have a sample changer equipped with a centrally arranged lift unit, the small amounts of torque occurring in the transverse direction are not significant enough to present a problem. Nevertheless, if a balance of the same type is designed for larger loads, a single lift unit arranged in the middle of a platform that is vertically movable relative to the weighing pan can prove to be a problem. When the relatively heavy platform is raised and lowered, transverse forces can occur that have an adverse effect on maintaining exact parallelism in the movement of the platform. Constraining the movement in conformance with the existing state of the art by means of several guiding elements arranged around a central lift unit requires a large amount of space and allows little flexibility. Furthermore, in the case of pneumatic as well as hydraulic lifting systems, preventive measures have to be taken against an accidental lowering of the platform from the lifted position, e.g., if there is a malfunction.